Hi Eddy!
Without a doubt, the best pizza at Walt Disney World can be found at
Via Napoli in Epcot. It's a relatively new restaurant and it has a fabulous atmosphere in addition to fantastic food, including pizzas. In fact, I'm not a fan of pizza at Walt Disney World in general, but I'm a huge fan of the pizza at Via Napoli. While it is relatively expensive in my opinion, I do believe it's worth every penny!
Be sure to make an Advanced Dining Reservation for the Table Service restaurant. It is very popular, especially for dinner. For the record, I have been able to walk up for lunch without a reservation a couple of times.
I'll point out that there is a window on the outside of the building where you can purchase a slice of pizza. However, that's not the same pizza that you can purchase inside at the restaurant and, in my opinion, is no where as good! Definitely go inside!
Enjoy planning your magical Walt Disney World Resort vacation!